A size measuring device has been suggested recently, in which an object is imaged to measure its size. The size measuring device recognizes an image of the object on a screen according to a feature value such as color, shape and the like, and extracts the shape of an object image being recognized. The size of the object is arithmetically determined according to the extracted shape of the object image. In order to extract the shape of the object image, the image is converted into a drawing image in which shape lines are used to express the image. The extraction is based on the shape lines included in the drawing image.
In a lung region extraction device disclosed in Patent Document 1, the object image is positioned at the center of the frame at the time of extracting the object image, to eliminate the shape lines contacting a boundary edge of the image. In the lung region extraction device, the shape of the object image is exactly extracted by not extracting lines other than the shape lines in the object image.